Introduction

The order Megaloptera is a small order of
insects in the infraclass Neoptera, division Endopterygota. The
Megaloptera are closely related to the Neuroptera (spongillaflies). The
Megaloptera comprise only two families, the Corydalidae (fishflies and
dobsonflies) and the Sialidae (alderflies).

Larvae of all species of
Megaloptera are aquatic and attain the largest size of all aquatic
insects. Larval Corydalidae are sometimes called hellgrammites or toe biters. The adult Corydalidae are large, having a wing span of up to 16 cm (Megaloptera = "large wing").

Life History

Females of this holometabolous order lay
elongate eggs in masses on vegetation overhanging the aquatic habitat,
on large rocks projecting from the water, or on bridge abutments. After
about a week at cool temperatures, eggs hatch at night and first-instar
larvae fall into the water. As young larvae swallow air, gas bubbles
form in their guts, possibly providing the buoyancy necessary to
transport to riffles first instars that land in pools. The metabolic
consequences of this air bubble are unknown for most species.
Megalopteran larvae go through 10-12 instars before crawling out of the
water onto shore to pupate. Some have been reported to pupate as far as
50 metres from the shore.

Most sialids have one- or two-ear
life cycles, whereas corydalids in cold mountain streams and in
intermittent streams may live for up to five years.

Habitat and Ecological preference

The larvae of the Corydalidae live in clear
waters and are predaceous. The Sialidae are also widely distributed but
are confined to temperate latitudes. The larvae tend to live in more
turbid waters, or at least those with silty or muddy substrates; they
prey upon smaller insects.

Some species of Megaloptera
inhabit temporary streams. In these ephemeral habitats, the female lays
her eggs on the surfaces of rocks on the dry stream bed. Upon hatching,
the larvae burrow into the bed to await the return of the water. The
larvae feed and grow while the stream is flowing and, as it begins to
dry up once more, bury themselves under large rocks on the stream bed
where they construct their pupal cells.

Corydalids are found in
well-oxygenated streams and lakes, as well as in productive ponds or
swamps where dissolved oxygen may be very low. Sialids occur in the
same broad habitat categories, but usually require muddy or silty
deposits and accumulated detritus.

Sialids are classified as burrowers, while corydalids are generally clingers or climbers.

Feeding

The larvae of both families of Megaloptera
are active predators, feeding on aquatic insects, annelids,
crustaceans, and mollusks. Little is known about the predatory behavior
of these groups, despite their conspicuous presence and apparently
voracious appetites. Some classify them as engulfers.

Indicator value

Larvae of all species of Megaloptera are
aquatic and attain the largest size of all aquatic insects. Larval
Corydalidae are sometimes called hellgrammites or toe biters. The adult
Corydalidae are large, having a wing span of up to 16 cm (Megaloptera =
"large wing"). Corydalids (fishflies and dobsonflies) are found in
well-oxygenated streams and lakes, as well as in productive ponds or
swamps where dissolved oxygen may be very low. Sialids (alderflies)
occur in the same broad habitat categories, but usually require muddy
or silty deposits and accumulated detritus (Kellogg, 1994, and Williams & Feltmate, 1992).

All are intolerant of pollution.
Although they do commonly occur in waters with pH levels near 5.5,
circumneutral or alkaline waters seem to have the largest populations. Sialis
is considered to be more tolerant than the corydalids but cannot
tolerate extreme conditions either. No species are recognized as good
indicator organisms (Mackie, 2001).